The Orange basin, located in
Southern Africa, covers almost 3% of the continent and spreads over four countries (Map 11
and Table 46).

Table 46 :Orange basin: areas and
rainfall by country

Country

Total area of the country (km2)

Area of the country within the basin (km2)

As % Of total area of basin (%)

As % of total area of country (%)

Average annual rainfall in the basin area (mm)

min.

max.

mean

Botswana

581730

71000

7.9

12.2

165

520

295

Namibia

824900

219249

24.5

26.6

35

415

185

Lesotho

30350

30350

3.4

100.0

575

1040

755

South Africa

1221040

575769

64.2

47.2

35

1035

365

For Orange basin

896368

100.0

35

1040

325

Rivers and discharges

The source of the Orange River is in
Lesotho. The river receives water from the Makhaleng tributary just before entering South
Africa. The Caledon tributary flows on the border between South Africa and the north of
Lesotho and flows into the Orange River further downstream in

South Africa. The average annual
runoff from Lesotho to South Africa is estimated at 4.73 km3/year, which is far
in excess of the country's water requirements.

Almost the entire plateau of South
Africa, representing over 48% of the area of the country, is drained by the Orange River
and its tributaries, though they contribute only about 22% of the total runoff of South
Africa. The Vaal is the major tributary of the Orange River and the average annual runoff
in the Vaal basin area is about 4.27 km3, of which 2.15 km3 is
exploitable. The average annual runoff of the Orange basin, excluding the Vaal, is
estimated at 7.59 km3, of which 5.76 km3 is exploitable.

The Molopo, which forms the border
between Botswana and South Africa, is a fossil river, which once flowed into the Orange
River. Now it receives most of its very occasional flows from its tributaries in the
northern Cape province of South Africa [61].

The Orange River forms the border
between the south of Namibia and South Africa. The most important tributary entering from
Namibia is the Fish River, on which the Hardap dam was constructed in 1972.

Irrigation potential and water
requirements

The irrigation potential in the
orange basin in Botswana is negligible because of a lack of renewable water
resources [64].

Namibia currently has access
to an agreed volume of at least 0.5 km3/year of water from the Orange River
[162]. The gross irrigation water requirement is estimated at 8500 m3/ha per
year in the present study. Literature gives figures of 33000 m3/ha per year
[163]. The difference can be explained by a difference in the assumed future irrigation
cropping pattern. As probably good soils alongside the rivers will be a limiting factor,
an irrigation potential of 25000 ha has been retained [*].

The soils of Lesotho, which
lies entirely in the Orange basin, are very poor [192]. The irrigation potential has been
estimated at 12500 ha [127]. Through the Lesotho highlands water project, it is planned to
transfer about 2.1 km3/year of water from Lesotho to South Africa while
enabling Lesotho to generate its own electricity.

The water available for agriculture
in the Orange basin in South Africa is estimated at 4.3 km3/year by the
year 2010 [190]. Table 47 summarizes the water resources, irrigation potential and water
requirements [190, 191, *].

Table 47: Irrigated areas, water
availability, water requirements and irrigation potential in the Orange basin in South
Africa

Sub-basin

Irrigated area (ha)

Actual water use (km3/yr)

Water available (km3/yr)

Irrigation water requirement (m3/ha.yr)

Irrigation potential (ha)

Vaal (C)

160000

1.366

1.770

14000

126400

Orange (D)

140000

1.413

2.488

11000

226100

Total for Orange basin in South Africa

300000

2.779

4.258

352500

For the whole of the Orange basin in
South Africa, the available water of 4.258 km3/yr would lead to an irrigation
potential of 352500 ha, using between 11000 and 14000 m3/ha of water per year
[*]. At the moment 300000 ha already benefit from irrigation, using less than 10000 m /ha
per year.

Table 48 below summarizes the
irrigation potential, water requirements and irrigated areas for the Orange basin.

The irrigation potential of 390 000
ha for the Orange basin should be considered as an upper limit from the point of view of
water availability. It would require agreed rules for the management of the river water by
Namibia and South Africa.

The South Interior is divided into
two separate basins, as shown in Figure 2. One is shared by Zimbabwe, Botswana and
Namibia. A major part of the Kalahari Desert is located in this basin. The other one is
shared by Angola and Namibia. Its total area represents 2.1% of the area of the continent
(Map 12 and Table 49).

Table 49: South Interior: areas
and rainfall by country

Country

Total area of the country (km2)

Area of the country within the basin (km2)

As % Of total area of basin (%)

As % of total area of country (%)

Average annual rainfall in the basin area (mm)

min.

max.

mean

Zimbabwe

760

24210

3.7

6.2

465

660

550

Botswana

581760

368780

57.1

63.4

270

670

405

Angola

1246700

53118

8.2

4.3

500

905

680

Namibia

824900

199718

30.9

24.4

275

580

410

For South Interior

645826

100.0

270

905

435

Rivers and discharges

The surface water resources of
Zimbabwe are estimated at 0.038 km3/year, of which 0.008 km3 is
still available for irrigation development after deducting quantities already used or
committed [216]. The annual runoff of the Mosupe and Mosetse rivers, located in Botswana,
is estimated at 0.055 km3. Most of the rivers are ephemeral. In Angola the
South Interior occupies 4% of the area of the country, but no information is available on
water resources. In the Namibian part of the basin there are only ephemeral rivers.

Irrigation potential and water
requirements

The area already developed or
planned for irrigation in Zimbabwe is about 250 ha [216]. The combination of
suitable land, which is already limited, and available water, which is even more of a
limiting factor. leads to an irrigation potential of 1100 ha, considering an irrigation
water requirement of 9500 m3/ha per year [*]. Should 18000 m3/ha per
year of water be used, as planned in the irrigation subsector review of Zimbabwe [216],
the area would have to be even less than 1100 ha.

The maximum irrigation potential for
Botswana in the South Interior has been estimated at 3950 ha, all of it being
located in the Makgadikgadi Pans in the eastern part of Botswana. About 1450 ha are
marginal land and would need major constructions for water development and storage [64].
In this study, 2500 ha have been retained for the irrigation potential. The Central
Kalahari Game Reserve in Botswana occupies a large area of the South Interior.

For Angola, no details are
available on the distribution of the irrigation potential over the country. It has been
estimated that 50000 ha are located in the South Interior (see the section The
Congo/Zaire basin) [*].

In Namibia, in the part of
the South Interior which is shared with Angola, a large area is occupied by the Etosha
National Park. Irrigation potential has been estimated at 400 ha [*]. The present study
estimates a water requirement of 5500 m3/ha per year. Other estimates,
considering a different irrigation cropping pattern, go as high as 34000 m3/ha
per year [163].

Table 50: South Interior:
irrigation potential, water requirements and areas under irrigation